Bo Boddicker, of Family Tree Care, cuts down an ash tree, Monday, March 9, 2015, in Des Moines, Iowa. Daunted by the cost and difficulty of stopping the emerald ash borer, many cities are choosing to destroy their trees before the insect can. Chain saws are roaring in towns where up to 40 percent of the trees are ashes, and rows of stumps line streets once covered by a canopy of leaves. (AP Photo/Charlie Neibergall) (The Associated Press)

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A crew from Family Tree Care cut down an ash tree, Monday, March 9, 2015, in Des Moines, Iowa. Daunted by the cost and difficulty of stopping the emerald ash borer, many cities are choosing to destroy their trees before the insect can. Chain saws are roaring in towns where up to 40 percent of the trees are ashes, and rows of stumps line streets once covered by a canopy of leaves. (AP Photo/Charlie Neibergall) (The Associated Press)

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Logs from a recently cut down ash tree sit in front of home, Monday, March 9, 2015, in Des Moines, Iowa. Daunted by the cost and difficulty of stopping the emerald ash borer, many cities are choosing to destroy their trees before the insect can. Chain saws are roaring in towns where up to 40 percent of the trees are ashes, and rows of stumps line streets once covered by a canopy of leaves. (AP Photo/Charlie Neibergall) (The Associated Press)

DES MOINES, Iowa – Since she was a child, the giant ash tree that towered over Rebecca Robinson's small home offered a cool refuge during sultry Midwest summer days. It was the same down her tree-lined neighborhood's block and throughout much of Waterloo, a leafy Iowa city that's home to about 4,000 ashes.

But work crews have toppled Robinson's tree and soon, nearly all of Waterloo's ash trees will be gone too — though many are perfectly healthy — as big cities and small towns from Pennsylvania to Colorado surrender to a small, shiny bug by preemptively eliminating a big part of their urban foliage.

With the loss of so many mature trees, some towns may not look the same again for decades.

"Is some places, you could have a pretty hostile environment," said Gary Johnson, a forestry professor in Minnesota, where thousands of lush trees are falling.

The emerald ash borer, which is native to Asia, was first spotted in the U.S. in 2002, when it showed up in the Detroit area. It devastated ash trees in Michigan and has spread to at least 21 other states as people haul firewood or other wood products from place to place.

Now, daunted by the cost and difficulty of stopping the insect, many cities are choosing to destroy their trees before the borer can. Chain saws are roaring in towns where up to 40 percent of the trees are ashes, and rows of stumps line streets once covered by a canopy of leaves.

About 50 million trees have been removed so far. With roughly 7 billion ash on public and private land in the U.S., the job has only begun.

"It was beautiful. It provided a whole lot of shade," said Robinson of the 75-foot tree in her yard. "I was sad to see it go, but on the other hand I didn't want it falling on my house."

Although the emerald ash borer nibbles on leaves, the trouble comes when the insect deposits larvae that burrow beneath tree bark, then zigzag along the tree, cutting off the flow of nutrients. Infected trees usually die within five years.

Whereas removal was the only option when the insect was discovered, city officials can apply newly developed insecticides.

Johnson, a professor at the University of Minnesota, pushes hard for preservation, noting the mature trees offer energy-saving summer shade and make neighborhoods more beautiful.

"There's been so much evidence that treating the ash trees is so much more economical than removing them," Johnson said.

The treatments, which average about $200 depending on size, must be done every three years. The cost compares to $1,000 or more to remove a tree. But some cities are reluctant to take on the many years of upkeep.

Johnson praised cities such as Milwaukee, which initially intended to cut down all 31,000 ash along streets and in parks but then decided to preserve all but about 5,000.

Chicago chose to remove 15,000 of its estimated 85,000 ash and treat the rest.

But in many places, the majority of the trees will fall.

In Des Moines, officials plan to save about 5,800 in conspicuous places, cut down 7,200 others and let about 34,000 in forests die on their own. Some residents are already preparing for a more barren feel this summer. On a street lined with 1920s bungalows, workers this week removed one massive tree that barely fit between the sidewalk and curb. A nearby block is lined with stumps.

Minneapolis is cutting down all of its ash trees on public property and planting new ones.